1. Field of Invention
This invention relates to a small size circuit breaker such as ones installed in distribution lines.
2. Description of Related Art
FIG. 8 shows the general construction of a circuit breaker (breaker for three-pole or phase wirings) and respective parts are housed in a housing made of a plastic molding consisting of a case 1 and a cover 2 covering the case 1. When the breaker is closed as shown, current flows through a stable contactor 3 used also as a terminal connected to an electric source, a movable contactor 4, a lead wire 5, a connector plate 6, an overcurrent tripping device 7, a conductor 8 and a terminal to a load.
The movable contactors 4 respectively are held in respective holders 10 of plastic moldings. These holders 10 are connected to each other by means of an open-and-close shaft 11 integrally formed. The open-and-close shaft 11 is fitted into a U-shaped groove (not shown) formed in a partition 1a situated between phases of the case 1 and rotatably held in the groove. In particular, the open-and-close shaft 11 fitted in the U-shaped groove has flanges of insulating barriers which are fitted in grooves formed in the circumference edges of the U-shaped groove.
The movable contactor 4 is driven to open and close the circuit by an open-and-close mechanism 12 through the holders 10. The open-and-close mechanism 12 is held at its set condition shown when a latch 14 held on a frame 13 provided with side plates at both sides of the movable contactor 4 engages with a latch receiver 15. As seen in FIG. 8, the latch receiver 15 has a clockwise from the latch 14 and is prevented from rotating by means of a claw 17 of a trip cross bar 16 engaged with the back face of the latch receiver 15.
In addition, the open-and-close shaft 11 fitted in the between-phase-partition 1a of the case 1 is pressed so as to prevent from floating-up or rising by a holder presser 18 formed on the frame 13 by outward being a part of the frame 13. The frame 13 is made of an iron plate piece and shaped in a U-shape as shown in FIG. 10. The frame 13 is threadly secured to a supporting board (not shown) of the case 1 by means of a bottom plate 13a connecting left and right side plates and attachment pieces 13b formed by cutting and bending parts of the side plates at the opposite end from the bottom plate 13a. A reference numeral 13c is a hole through which a support shaft of the latch receiver 15 passes and 13d is an open hole or groove through which a support shaft of the latch 14 and the like passes.
Under this condition, flowing overload current and short-circuit current activates the overcurrent tripping device 7 to rotate the trip cross bar 16 counterclockwise so as to disengage the claw 17 from the latch receiver 15. Accordingly, the latch receiver 15 rotates clockwise and disengages from the latch 14. The movable contactor 4 opens and moves to the position shown by broken lines due to a motion of the open-and-close mechanism 12 and stops at the position above.
By the way, it is necessary to keep or hold the movable contactor 4 with a predetermined distance of separation when the circuit breaker open. Consequently, according to the conventional technology, a stopper 19 is formed on the cover 2 as shown or a stopper on the frame 13 supporting the open-and-close mechanism 12. The movable contactor 4 strokes such stopper when the contactor reaches its end position of a separation travel. However, the movable contactor 4 deforms when it is considerably heated due to a large-current breaking of the circuit breaker and then strikes a stopper. According to the conventional breaker provided with a frame 12 and a stopper formed on the frame, there is a danger that the latch 14 is disengaged from the latch receiver 15 or the latch receiver 15 is disengaged from the claw 17, resulting in a tripped condition of the circuit breaker.
A proposed solution to the aforementioned problem is described Japanese Utility Model Unexamined Application No.59-178843, in which an arm used also as an insulation barrier is integrally formed on the open-and-close shaft 11 to be fitted in the U-shaped groove of the case partition; a groove containing the arm is formed in the circumference edge of the groove; and the arm engages with a circumferential wall of the groove when the movable contactor opens in order to restrict an open position of the movable contactor.
Such construction of the breaker will be explained again briefly with reference to FIG. 9. Here, FIG. 9(A) is a side view of the U-shaped groove portion and FIG. 9(B) is a sectional view along the B--B line in FIG. 9(A). It is apparent that the same reference numerals used to the parts in FIG. 8 are used to the corresponding parts of FIGS. 9(A) and (B). In the drawings, 1a shows a between-phase partition of the case 1, 20 is a U-shaped groove formed in the between-phase partition, 21 is a groove formed on the circumferential edge of the U-shaped groove 20, 11 is an open-and-close shaft fitted in the U-shaped groove 20, 22 is an arm used also as an insulation barrier formed integrally to the open-and-close shaft 11 and contained in the groove 21, and 23 is a piece used to fill up spaces on the upper portions of the grooves 20, 21 and to lengthen an insulation distance. In a condition in which the movable contactor opens as shown, the arm 22 engages, through its end, with the circumference wall 21a of the groove 21 so as to restrict an open position of the movable contactor.
According to the construction above, the arm 22 has a thicker wall than that of a mere insulation barrier so as to make the arm bearable to a large striking force or impact when the movable contactor is opened. The thickness or width size of the groove 21 containing the arm 22 is limited because the groove is formed in a range of the thickness measurement of the between-phase partition 1a. The arm 22 is also limited in its thickness. As a result, the end portion of the arm 22 touching the circumference wall 21a is apt to be collapsed and a touching face of the circumference wall 21a is easy to become depressed or caved in.
When a collapse is generated on the arm 22 and a cave in is formed on the circumferential wall 21a, sliding resistance between the arm 22 and the circumferential wall 21a increases when the open-and-close shaft 11 is rotated along the opposite direction to rotate the movable contactor from its open condition to a closed condition. Rotation resistance of the open-and-close shaft increases because abrasion dust or powder of the arm 22 and the partition wall 1a collects between the open-and-close shaft 11 and the U-shaped groove 20, resulting in an increase of necessary load. In addition, because the stoppage position of the arm 22 changes, an open position of the movable contactor 4 and a rotation angle of the holder 10 changes, so that the movable contactor 4 interferes disadvantageously with construction parts of the open-and-close mechanism. Consequently, operations of internal accessories, for example supplemental switches to be operated by rotations of the holder 10 are adversely affected.
As described above, according to the conventional example shown in FIG. 9, the width of the groove 21 is limited in selection, so that the arm 22 fills the full width of the groove 21 as shown in FIG. 9(B) and there is substantially no space allowance in the width direction of the groove 21. Consequentially, the piece 23 to be fitted in the between-phase partition 1a can not enter the gap G between the side wall of the groove 21 and the side face of the arm 22, resulting in its covering only the upper face of the arm 22 and the open-and-close shaft 11 as shown. As a result, a triangle gap 24 is generated between the piece 23 and the open-and-close shaft 11 as shown in FIG. 9(A) depicting a side view of the U-shaped groove 20 resulting in some troubles about insulation between phase when big current, such as short-circuit current is open. In addition, it is difficult to make, by a plastic molding process, the piece 23 having a sharp end filling-up the gap 24, so that there is a possibility of chipping the sharp end during a handling of the piece if it is managed to make the sharp end.
Again, as shown in FIG. 8, the open-and-close shaft 11 is held under pressure so as not to rise by a holder presser 18 formed on a part of the frame 13 supporting the open-and-close mechanism 12. According to this construction, when the breaker opens and closes, impact generated in the movable contactor 4 upon closure of the breaker is transferred to the frame 13 through the open-and-close shaft 11. Consequently, the engagement among the latch 14 supported by the frame 13, the latch receiver 15 and the claw 17 is disengaged resulting in the failure of the failure to close. In order to avert this danger, the frame 13 is strengthened by increasing the thickness and width thereof. The force of the return spring 25 of the trip cross bar 16 may also be increased. Nevertheless, the more the frame 13 strengthens, the more a size of the case 1 enlarges disadvantageously, and when the force of the return spring 25 enlarges, the overcurrent tripping characteristic deteriorates.
Concerning FIG. 8, current flows through the stable contactor 3 and the movable contactor 4 in opposite directions. Consequently, magnetic forces repelling each other are generated in both the currents. The circuit breaker is constructed so as to use the magnetic repulsion, when a large current, such as a short-circuit current flows, driving rapidly the movable contactor 4 along its opening direction. However, very large bending stress making the holder presser 18 a fulcrum is generated in the open-and-close shaft 11. Accordingly, in order to prevent the shaft 11 from being broken by the bending stress, it is necessary to thicken or increase a diameter of the shaft or use a strong material of the shaft resulting in a large and expensive case 1.